1. Field of the Invention
The present invention relates to a method for making n-type semiconducting diamond or diamond film by use of chemical vapor deposition.
2. Description of the Prior Art
Diamond exhibits the general properties as follows. First, it is chemically stable. When considered in the view of thermodynamics, diamond is quasi-stable, but it is chemically more stable than any other material on earth, so that its industrial application is actively pursued. In addition, diamond is very high in thermal conductivity. By virtue of this property, diamond is utilized as a heat-absorbing and emitting material in the semiconductor industry. Diamond, also is of the highest degree of hardness among all the materials on earth. So, there have been many attempts to utilize diamond in tools and, in fact, many tools made of diamond are commercially available. Further, diamond has a significant advantage of readily emitting electrons from its surface. In this regard, diamond is now believed to be a promising material in field emission display field applications.
Let us turn now to the semiconducting properties and application fields of diamond.
First, if boron compounds are added during the preparation of diamond, a p-type semiconductor can be obtained. n-Type semiconducting thin films are reportedly made by doping phosphorous (P) or taking advantage of an ion-implantation technique during the preparation of diamond. However, this is poor in reproductivity and thus, it is now difficult to fabricate n-type semiconductor devices with diamond.
Diamond is fifty times greater in dielectric break down voltage and 5.5 times greater in dielectric constant as compared with GaAs. In addition, diamond is 2,500 times greater in theoretical output and twenty times greater in thermal conductivity in as compared with Si. Further, diamond is superior in saturation carrier velocity as well as in the resistance to electric fields. These thermal and semiconducting properties of diamond make it unnecessary to transform line voltage into input voltage in a high voltage transistor, enabling the magnitude of power supply to be minimized. The Si MOSFETs in current use have too small a capacitance of current flow and exhibit a drain-to-source breakdown voltage of only 10-15 V. These problems can be solved by employing diamond. In addition to maintaining high voltage, diamond exhibits high forward transconductance and superior high-frequency performance, so that the package can be largely reduced in size (Yoon-ki Kim, "Study on the heteroepitaxy growth of diamond thin film by microwave plasma chemical vapor deposition method", a doctoral dissertation, 1997, p13, Dept. of Materials science and engineering, Korea Advanced Institute of Science and Technology).
There are various techniques of artificially making diamond, including the high temperature-high pressure method, shockwave method and CVD method ("Properties and Applications of Diamond", John Wilks and Eileen Wilks, Butterworth heinemann, 1991, pp 7-27).
The methods for making p-type diamond are now well known to those skilled in the art, whereby the diamond is stably produced. As a p-type impurity, boron (B) is used the most (see "Field emission from p-type polycrystalline diamond films", D. Hong and M. Aslm, J. Vac. Sci. Technol. B 13(2), Mar/Apr 1995, pp427-430; "Boron doped diamond films: electrical and optical characterization and the effect of compensating nitrogen", R. Locher, J. Wangner, F. Fuchs, C. Wild, P. Hiesinger, P. Gonon, p. Koidl, Materials science and engineering B29, 1995, pp211-215; "Effect of annealing in air on electrical resistance of B-doped polycrystalline diamond films", Koichi Miyata, David L. Dreifus, Jpn. J. Appl. Phys., 33, 1994, pp4526-4533).
Many attempts have been made to make n-type diamond but no reliable methods are firmly established. As impurities for n-type diamond, N, P and Li are used. Using these impurities, an ion implantation method, a forced diffusion method and a CVD method have been developed for making n-type diamond, thus far.
The ion implantation method, however, has a significant disadvantage in that, when the n-type impurities are penetrated into the crystal lattice of diamond, damage occurs on its surface and is not restored even by annealing techniques. An n-type diamond is not reported to be successfully made by CVD method. The forced diffusion method is also known to be disadvantageous in production cost and its effect is insignificant ("Diamond: Electronic properties and applications", Lawrence S. Pan, Don R. Kania, Kluwer Academic Publishers, 1995, pp153-168; "Doping diamond for electronic applications", R. Kalish, Proceedings of the international diamond symposium, Seoul, 1996, pp 45-50; "Prospective n-type impurities and methods of diamond doping", G. popovici, M. A. Prelas, Diamond and related materials 4 (1995), pp 1305-1310).